Your string mop got stinky because you left it in a dark corner — wet — and now it is full of odor-causing bacteria. There are numerous tactics to deodorize the mop, but you need to keep it in a dry spot should youn’t want it to get smelly again.
Kill the Cooties With Chlorine
Chlorine bleach is the number-one household disinfectant because it functions. Put it to work by mixing a cup of bleach in 3 liters of warm water and immersing your odorous cleaning implement — the mop — at it for about 15 minutes. After the answer has had a change to circulate through the strings, then hang the mop in a dry place and let it air dry. To prevent a recurrence, constantly hang the mop — do not let the strings rest on the ground for an extended period, since that motivates moisture to form underneath.
Bleach is a strong-smelling, corrosive cleaner that you may prefer to avoid, if at all possible. If so, mix 2 or 3 cups of baking soda using 1/2 to 1 cup of salt in 3 liters of warm water, and let the mop stand in it for half a day. You may also disinfect the mop with vinegar — 1 cup per gallon of water should do it. Whatever you use, it is essential to dry the mop out to stop bacteria from growing again.